The present invention relates to a lateral double-diffused MOS transistor and a manufacturing method therefor. More particularly, the invention relates to a lateral double-diffused MOS transistor having high breakdown voltage and low on-state resistance characteristics as well as a manufacturing method therefor.
In recent years, with the trend toward more multifunctional electronic equipment, semiconductor devices to be used therein have been diversified, confronting demands for higher breakdown-voltage, higher power, smaller size and lower power consumption. Achieving lower power consumption needs transistors of lower on-state resistance.
FIG. 6 shows the structure of a common lateral double-diffused MOS transistor. This lateral double-diffused MOS transistor is an N-channel type MOS transistor in this example and includes a lightly doped N-well diffusion region 102 serving as a drift region formed on a P-type silicon substrate 101. A P-body diffusion region 103 for forming a channel is formed on a surface within the lightly doped N-well diffusion region 102. A gate electrode 105 is provided in such a position as it covers from part of the P-body diffusion region 103 to part of the N-well diffusion region 102 located outside the diffusion region via gate oxide 104. An N+ source diffusion region 106 and an N+ drain diffusion region 107 are formed on top of the P-body diffusion region 103 and top of the N-well diffusion region 102, respectively, both of which correspond to both sides of the gate electrode 105. A region of the P-body diffusion region 103 which is located just under the gate electrode 105 and which is sandwiched by the N+ source diffusion region 106 and the N-well diffusion region 102 forms the channel. Also, the P-body diffusion region 103 is short-circuited to the N+ source diffusion region 106 via a P+ backgating diffusion region 108 and unshown interconnections, thereby preventing operation of parasitic NPN.
The lateral double-diffused MOS transistor is required to have high breakdown voltage and low on-state resistance. The breakdown voltage depends on the horizontal distance between the P-body diffusion region 103 and the N+ drain diffusion region 107 (length of drift region), and on the concentration of the N-well diffusion region 102. That is, the breakdown voltage becomes higher with increasing length of the drift region and decreasing concentration of the N-well diffusion region 102. However, lower on-state resistance, which is another necessary performance, necessitates shorter drift region and higher concentration of the N-well diffusion region 102. As a result of this, the relationship between breakdown voltage and on-state resistance is a trade-off. Moreover, the demand for smaller size makes it unacceptable to make a choice of elongating the drift region to increase the breakdown voltage.
In contrast to this, a DDD (Double Diffused Drain) structure as shown in FIG. 7, which is rather commonly used, and a structure as shown in FIG. 8 are proposed in JP H11-340454 A. It is noted that component elements in FIGS. 7 and 8 corresponding to those of FIG. 6 are designated by reference numerals obtained by addition of 100 and 200 for FIGS. 7 and 8, respectively, to those of FIG. 6. The structures shown in FIGS. 7 and 8 each have an N diffusion region (higher in concentration than N well diffusion region 202, 302 and lower in concentration than N+ drain diffusion region 207, 307) 209, 309 which is provided so as to surround the N+ drain diffusion region 207, 307, respectively. In these structures, since concentrations in vicinities of the N+ drain diffusion region 207, 307 with respect to the horizontal direction out of the drift region is set higher, indeed the on-state resistance becomes somewhat lower than in the structure of FIG. 6, but the breakdown voltage still becomes lower.